Centrifugal fan

ABSTRACT

A centrifugal fan includes an impeller that rotates inside the elongated cylindrical housing and a motor for driving the impeller. The impeller and the motor are arranged tandem in the axial direction. The impeller includes a blade portion having plural blades elongated in the axial direction, and the blades are arranged at a predetermined pitch in the circumferential direction. When the impeller rotates, air is taken in through a suction opening provided to the tip portion in the axial direction of the housing and is blown out through an outlet opening that is provided to a part in the circumferential direction of the housing. A bearing portion that constitutes the motor is a sleeve bearing including a shaft member and a sleeve having a cylindrical shape that engages the shaft member with a clearance. One of the shaft member and the sleeve is fixed to a rotational member, while the other is fixed to a base member. One of the shaft member and the sleeve is made of a ceramic, while the other is made of a ceramic or a metal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a centrifugal fan for cooling embeddedin electronic equipment.

2. Description of the Prior Art

Recently, cooling fans embedded in electronic equipment such as apersonal computer have become smaller and thinner. Conventionally, mostelectronic equipment incorporates a cooling fan of an axial flow type.It is because the axial flow type cooling fan typically has a structuresuitable for a thin shape or a low profile. In contrast, althoughcentrifugal fans have an advantage of having higher static pressure thanaxial fans, they also have a disadvantage of difficulty in reducing adimension in the axial direction for a low profile compared with axialfans. In order to realize a centrifugal fan that can be embedded in lowprofile electronic equipment such as a note type personal computer, itis possible to adopt a structure in which a dimension in the radialdirection of an impeller is reduced while a length in the axialdirection of the same is increased, and the impeller is rotated at ahigh speed.

FIG. 6 is a cross sectional view in the axial direction showing astructure of a long thin centrifugal fan. In addition, FIG. 7 is a crosssectional view in the direction perpendicular to the rotational axis ofan impeller in a typical centrifugal fan. Although shapes and scales ofeach element are not identical between FIG. 6 and FIG. 7, elementshaving the same functions are denoted by the same reference numeral. Thelong thin centrifugal fan shown in FIG. 6 includes a housing 101 havinga substantially cylindrical shape elongated in the axial direction,which houses an impeller 102 and a motor 103 for rotating the impeller102. The impeller 102 is disposed at the top side (the right side inFIG. 6), in the axial direction within the housing 101, while the motor103 is disposed at the bottom side (the left side in FIG. 6) in theaxial direction within the housing 101.

The impeller 102 includes a blade portion 104 (at the top side) havingplural blades 104 a of a shape elongated in the axial direction that arearranged at a predetermined pitch in the circumferential direction and abottom end portion 105 having a substantially cylindrical shape forsupporting the blade portion 104. The tip of the blade portion 104 isprovided with a ring-like linking portion 106 for linking and supportingtip portions of the plural blades 104 a. When the impeller 102 rotates,external air is taken in through a suction opening 107 provided to thetip portion in the axial direction of the housing 101 as illustrated byan arrow IN. Then, the air is blown out through an outlet opening(corresponding to 108 in FIG. 7) externally that is provided to a partin the circumferential direction of the housing 101 (as illustrated byan arrow OUT in FIG. 7).

The motor 103 includes a rotor yoke 111 and a rotor magnet 112 thatcorresponds a rotational member rotating with the impeller 102, a statorarmature 113 and a fixed shaft 114 on a stational side, a pair of ballbearings 115, and a base member 116 for fixing the fixed shaft 114 tothe housing 101. The bottom end portion 105 of the impeller 102 isengaged with and fixed to the outer surface of the rotor yoke 111 thatis a cylindrical magnetic member. A plurality of rotor magnets 112 isarranged in the circumferential direction at a predetermined pitch onthe inner surface of the rotor yoke 111 at the middle portion in theaxial direction, and a pair of outer ring portions of the ball bearings115 is disposed separately in the axial direction at both sides of therotor magnets 112.

The stator armature 113 is fixed substantially at the middle portion inthe axial direction of the fixed shaft 114 so as to be opposed to therotor magnets 112 with a constant gap. In addition, a pair of inner ringportions of the ball bearings 115 is fixed to the fixed shaft 114separately in the axial direction at both sides of the stator armature113. The bottom side of the fixed shaft 114 is engaged with and fixed toa center hole of the base member 116 so that an axis of the fixed shaft114 agrees the rotation axis of the rotor yoke 111.

When the stator armature 113 is driven (excited) to generate a revolvingmagnetic field, the rotor magnet 112, the rotor yoke 111 and theimpeller 102 are rotated as one unit body responding to the revolvingmagnetic field. Then, as described above, the rotation of the impeller102 causes air flow that is taken in through the suction opening 107 atthe tip portion in the axial direction of the housing 101 and is blownout through an outlet opening formed at a part in the circumferentialdirection of the housing 101.

The long thin centrifugal fan having the above-mentioned structure isrequired to have a large length of the impeller in the axial directionand to be rotated at a high rotation speed in order to compensate lowcapacity of air blowing due to a small diameter of the impeller. Forexample, dimensions of the impeller is designed so that its radius r andits length in the axial direction h satisfies the relationship2r<=h<=20r, and the rotation speed of the impeller is set to a value of15,000 revolutions per minute or more. In this case, since the diameterof the impeller is small, noise (wind noise) due to the rotation of theimpeller is not so large despite of the high rotation speed. Indeed, itsnoise is smaller than an ordinary centrifugal fan.

However, the desire to realize lower noise level has grown recentlyparticularly in portable electronic equipment such as a note typepersonal computer. Therefore, there is a tendency that an acceptablelevel of noise generated by a cooling fan becomes lowered. Concerningthe long thin centrifugal fan having the above-mentioned structure,noise generated by ball bearings has become larger than the wind noisegenerated by the impeller.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a centrifugal fan thathas a novel structure of bearing so as to suppress generation of noise.Another object of the present invention is to reduce further a dimensionin the radial direction of the long thin centrifugal fan.

According to a first aspect of the present invention, a centrifugal fanis provided that includes an impeller including a blade portion havingplural elongated blades arranged at a predetermined pitch in thecircumferential direction and a motor for rotating the impeller. Theimpeller and the motor are arranged tandem in the axial direction. Abearing portion that constitutes the motor is a slide bearing includinga shaft member and a sleeve having a cylindrical shape that engages theshaft with a clearance. One of the shaft member and the sleeve is madeof a ceramic, and the other is made of a ceramic or a metal.

According to this structure, since a slide bearing (also referred to asa sleeve bearing) is used for the bearing portion, noise due to rotationcan be reduced compared with the conventional ball bearing. Furthermore,since at least one of the shaft member and the sleeve that constitutethe slide bearing is made of a ceramic, a good durability can beobtained. Note that it is preferable that both of them are made of aceramic for a long life because seizing up occurs hardly between them.If one of them is made of a ceramic and the other is made of a metal,the sliding surface of the metal is preferably hardened by a surfacetreatment or coating so that a long life is obtained.

According to a second aspect of the present invention, a reduceddiameter portion is formed on the shaft member at a middle portion inthe axial direction of the sliding portion with the sleeve, the reduceddiameter portion having a diameter a little smaller than other portions.In this way, an area of the sliding surface is reduced so that a bearingloss due to sliding friction can be reduced. Note that although it ispossible to cut a part of the inner surface of the sleeve instead of theshaft member for reducing an area of the sliding surface, cutting of theshaft member is easier than cutting of the inner surface of the sleeve.

According to a third aspect of the present invention, means for keepinglubricating oil that is supplied to the sliding surfaces of the shaftmember and the sleeve are attached to the shaft member or the sleeve.For example, a felt member having a ring-like shape impregnated withlubricating oil can be attached to the shaft member or the sleeve as themeans for keeping lubricating oil. When the lubricating oil is suppliedto and is kept on the sliding surfaces of the shaft member and thesleeve, a bearing loss due to a friction on the sliding surface can bereduced.

According to a fourth aspect of the present invention, the shaft memberis fixed to a rotational member, and the sleeve that retains the shaftmember in a rotatable manner is fixed to the housing directly or via abase member. This structure is a so-called rotational shaft (or fixedsleeve) structure, and the number of components can be reduced by fixingthe sleeve to a base member (that is fixed to the housing) directly.This means a structure in which a sleeve and a sleeve holder are made asone unit member. It is particularly preferable to make such a memberusing a ceramic. In addition, as the sleeve holder is attached to thehousing directly, the number of components can be reduced further. Thismeans a structure in which the base member, the sleeve holder and thesleeve are made as one unit member.

According to a fifth aspect of the present invention, a sleeve made of aceramic is fixed to the rotational member, and a shaft member is fixedto the housing directly or via a base member. This structure is aso-called rotational sleeve (or fixed shaft) structure, and the numberof components can be reduced by fixing the shaft member to a base member(that is fixed to the housing) directly. In addition, the number ofcomponents can be further reduced by attaching the shaft member to thehousing directly. This means a structure in which the base member andthe shaft member are made as one unit member. Note that although asleeve bearing that uses a sleeve made of a sintered metal impregnatedwith lubricating oil is used conventionally, it is difficult to use sucha sleeve in this case of rotating sleeve. It is because that when thesleeve is rotated, the lubricating oil is moved to the outer side bycentrifugal force so that the sliding surface of the inner side will belack of the lubricating oil. In contrast, there is not such a problem ifa sleeve made of a ceramic is used as a ceramic bearing so that thestructure of the rotating sleeve can be realized easily.

According to a sixth aspect of the present invention, the top side ofthe shaft member is disposed within the inner space of the blade portionof the impeller. When the top side of the shaft member is positionedwithin the inner space of the blade portion of the impeller, a dimensionin the axial direction of the entire centrifugal fan can be reduced.

According to a seventh aspect of the present invention, the bottom sideof the shaft member is a free end without a thrust bearing. According tothis structure, a member (thrust plate, for example) for a thrustbearing can be eliminated, and generation of noise due to friction atthe thrust bearing portion as well as wear of the tip portion of theshaft member on the bottom side can be avoided. Note that it is possibleto support the bottom side of the shaft member by using air dumping oroil dumping.

According to an eighth aspect of the present invention, among elementsthat constitute the motor, a torque generating portion including anarmature and a field magnet, and a bearing portion are arranged tandemin the axial direction. According to this structure, a dimension in theradial direction of the centrifugal fan can be reduced. Namely, if thetorque generating portion and the bearing portion are arrangedsubstantially at the same position in the axial direction in a coaxialmanner, an outer diameter of the motor depends on a total sum of theouter diameter of the bearing portion and a size (thickness) of thetorque generating portion in the radial direction. In contrast, if thetorque generating portion and the bearing portion are arranged tandem inthe axial direction, the outer diameter of the motor depends on eitherthe outer diameter of the bearing portion or the size (outer diameter)of the torque generating portion that is larger one than the other.Therefore, it is possible to reduce the outer diameter of the motor,thereby reducing further the dimension in the radial direction of thecentrifugal fan. A combination of this structure and the sleeve bearingutilizing a ceramic member as described above can reduce the number ofcomponents so that the dimension in the radial direction can be furtherreduced.

According to a ninth aspect of the present invention, the rotationalmember rotates integrally as one element, the bearing portion isdisposed at a barycenter of the element or at vicinity of thebarycenter. This structure facilitates stabilization of rotation of theimpeller. Namely, vibration of the impeller accompanying rotationthereof can be reduced, and a load to the bearing portion can besuppressed so that a life of the bearing portion can be increased. Thisstructure is useful particularly in the case where the impeller isrotated at a high rotation speed. Note that the structure that thebearing portion is disposed at a barycenter of the element or atvicinity of the barycenter means a structure that an intermediate pointin the axial direction of the slide bearing (sleeve bearing) is disposedat a barycenter or at vicinity of the barycenter.

According to a tenth aspect of the present invention, a centrifugal fanis provided that includes an impeller including a blade portion havingplural blades arranged at a predetermined pitch in the circumferentialdirection and a motor for rotating the impeller. The impeller is rotatedat a rotation speed more than or equal to 15,000 revolutions per minute,a bearing portion that constitutes the motor is a slide bearingincluding a shaft member and a sleeve having a cylindrical shape thatengages the shaft member with a clearance, one of the shaft member andthe sleeve is made of a ceramic, and the other is made of a ceramic or ametal.

According to this structure, since a slide bearing (also referred to asa sleeve bearing) is used for the bearing portion, noise due to rotationcan be reduced compared with the conventional ball bearing even at highspeed rotation. Furthermore, since at least one of the shaft member andthe sleeve that constitute the slide bearing is made of a ceramic, agood durability can be obtained even the high speed rotation ismaintained. When the present invention is embodied, a small centrifugalfan can be rotated silently and at high speed. Therefore, a centrifugalfan can be downsized while increasing quantity of airflow and staticpressure thereof. Note that it is preferable that both of them are madeof a ceramic for a long life because seizing up occurs hardly betweenthem. If one of them is made of a ceramic and the other is made of ametal, the sliding surface of the metal is preferably hardened by asurface treatment or coating so that a long life is obtained. Inaddition, when using a ceramic, liquid lubricant can be eliminated,thereby a seal mechanism for the liquid lubricant can be eliminated sothat the structure can be simplified.

According to a eleventh aspect of the present invention, the sleeve andthe shaft member are both made of the same type of ceramic. Although aceramic is an insulator, generation of static electricity due toelectrification of the surface can be prevented by using the same typeof ceramic for the sliding surfaces. If static electricity is generated,the bearing may adsorb dust or micro particles, which may deteriorateperformance of the bearing. The present invention can be preferably usedparticularly in an environment where a lot of dust or the like exists.

According to a twelfth aspect of the present invention, a plurality ofshallow grooves is formed on at least one of the inner surface of thesleeve and the outer surface of the shaft member. The shallow groovescan be linear grooves extending in the vertical direction (axialdirection) or grooves having a herringbone shape, a screw-like shape (aspiral shape) or a curved shape. As a plurality of grooves having theseshapes is arranged, pressure varies along the axial direction so thatstiffness of the bearing is enhanced at a part of high pressure whenrelative rotation between the sleeve and the shaft member is generated.Therefore, contact between the sleeve and the shaft member can beprevented, and stability when an external force is applied can beincreased. Note that a set of grooves may be provided, or plural sets ofgrooves may be provided separately in the axial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a structure of a long thincentrifugal fan according to a first embodiment of the presentinvention.

FIGS. 2(a) and 2(b) show partial cross sections of the bearing portionaccording to variations of the embodiment shown in FIG. 1.

FIGS. 3(a) and 3(b) show partial cross sections of the bearing portionaccording to other variations of the embodiment shown in FIG. 1.

FIG. 4 is a cross sectional view showing a structure of a long thincentrifugal fan according to a second embodiment of the presentinvention.

FIG. 5 is a cross sectional view showing a structure of a long thincentrifugal fan according to a third embodiment of the presentinvention.

FIG. 6 is a cross sectional view showing a structure of a conventionallong thin centrifugal fan.

FIG. 7 is a cross sectional view in the direction perpendicular to theaxis of a housing and an impeller in a typical centrifugal fan.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to attached drawings. Note that when referring toa relationship of positions or directions of structural elements in thefollowing description, they mean a relationship of positions ordirections in a drawing and do not mean a relationship of positions ordirections in a state being incorporated in actual equipment.

Embodiment 1

FIG. 1 is a cross sectional view showing a structure of a long thincentrifugal fan according to a first embodiment of the presentinvention. This long thin centrifugal fan includes a housing 11 having asubstantially cylindrical shape elongated in the axial direction, whichhouses an impeller 12 and a motor 13 for rotating the impeller 12. Theimpeller 12 is disposed mainly at the top side (the right side inFIG. 1) in the axial direction within the housing 11, while the motor 13is disposed at the bottom side (the left side in FIG. 1) in the axialdirection within the housing 11.

The impeller 12 includes a blade portion 14 (at the top side) havingplural blades of a shape elongated in the axial direction that arearranged at a predetermined pitch in the circumferential direction, anda bottom end portion 15 having a substantially cylindrical shape forsupporting the blade portion 14. The tip of the blade portion isprovided with a ring-like linking portion 16 for linking and supportingtip portions of the plural blades. When the impeller 12 rotates,external air is taken in through a suction opening 17 provided to thetip portion in the axial direction of the housing 11 as illustrated byan arrow IN. Then, the air is blown out through an outlet opening thatis provided to a part in the circumferential direction of the housing11.

The impeller 12 having a shape elongated in the axial direction has adiameter less than or equal to 25 millimeters. In addition, a radius rand a length h in the axial direction of the impeller satisfies therelationship of 2r<=h<=20r. Since the impeller 12 has such an elongatedshape, a long thin centrifugal fan is realized that can be incorporatedin low profile equipment. In addition, since it is a centrifugal fan, ithas higher static pressure than an axial fan and is suitable for beingembedded in compact electronic equipment of a high density of mountingcomponents. In addition, the impeller 12 is rotated at a rotation speedmore than or equal to 15,000 revolutions per minute, or at a relativespeed more than or equal to 0.8 meters per minute between the shaft andthe bearing surface. Such a high rotation speed of the impeller 12secures sufficient quantity of airflow even by the long thin centrifugalfan with a reduced dimension (particularly in the radial direction).

The motor 13 includes a stator armature 21, a rotor magnet 22, a rotoryoke 25, a sleeve 31 and a sleeve holder 32 that constitute a sleevebearing as a slide bearing, a rotational shaft 33 and other members. Therotor yoke 25 that is a rotational member is provided with step portionsso that its diameter decreases from the bottom side to the top side bythree steps. In other words, the rotor yoke 25 has a large diameterportion 25 a, a medium diameter portion 25 b and a small diameterportion 25 c. For example, a plurality of the rotor magnets 22 isarranged on the inner surface of the large diameter portion 25 a of therotor yoke 25 at a predetermined pitch in the circumferential direction,and each of the rotor magnets 22 is opposed to the stator armature 21with a constant gap.

The bottom end portion 15 of the impeller 12 is engaged with and fixedto the outer surface of the medium diameter portion 25 b and the smalldiameter portion 25 c of the rotor yoke 25. The rotational shaft 33 thatis a rotatable shaft portion is fit in and fixed to the small diameterportion 25 c of the rotor yoke 25. The rotational shaft 33 is retainedby the cylindrical sleeve 31 in a rotatable manner, so that therotational shaft 33, the rotor yoke 25 and the impeller 12 are rotatedas one unit body. The sleeve 31 is engaged with the inner surface of thesleeve holder 32 and is fixed to the same.

In this embodiment, both the rotational shaft 33 and the sleeve 31 aremade of a ceramic. For example, alumina, silicon nitride, artic,zirconia or the like can be used as the ceramic material. It is possibleto use a ceramic for making one of the rotational shaft 33 and thesleeve 31 and use a metal for making the other. In this case, it ispreferable to use a martensitic stainless steel as the metal materialfor example, and to perform a surface hardening process (nitridingtreatment, for example) or DLC (diamond-like carbon) coating so thathardness of the surface (sliding surface) is increased.

The sleeve holder 32 has a cylindrical shape with an opening at the topside, and its bottom end portion is fixed to a center through-hole of abase member 24 that is a stational member by press fitting. The basemember 24 is fixed to the inner wall at the bottom side of the housing11 made of a resin (or a metal). The inner surface of the sleeve holder32 at the bottom side is provided with a thrust plate 34 made of a metalor a ceramic to which the bottom end portion of the rotational shaft 33can abut. Thus, a thrust bearing is constituted. If the rotational shaft33 is made of a ceramic, it is preferable that the thrust plate 34 isalso made of a ceramic or a metal having a hardened surface by theabove-mentioned surface hardening process or the like. Alternatively, itis possible to keep the bottom end portion of the rotational shaft 33 asa free end so that the thrust plate 34 (the thrust bearing) can beeliminated. In this case, air dumping or oil dumping may be used forretaining the bottom end portion of the rotational shaft 33. Inaddition, a seal member is provided for sealing a ring-like gap betweenthe opening of the sleeve holder 32 at the top side and the rotationalshaft 33. Thus, it is possible to prevent dust from entering the insideof the sleeve holder 32.

FIGS. 2(a)-2(b) and 3(a)-3(b) show partial cross sections of the bearingportion according to some variations of the embodiment shown in FIG. 1.In a structure shown in FIG. 2(a), the sleeve 31 and the sleeve holder32 in the structure shown in FIG. 1 are made as one unit member(hereinafter, it is simply referred to as a sleeve 31). In other words,the sleeve holder 32 is eliminated, and the bottom end portion of thecylindrical sleeve 31 is directly fixed to the base member 24.Furthermore, it is possible to make the base member 24 and the sleeve 31as one unit member, which is directly fixed to the housing 11. It ispreferable to use a ceramic as a material of these unit members. Inaddition, the seal that is necessary in the structure shown in FIG. 1 isnot necessary in the structure shown in FIG. 2(a). Furthermore, thethrust plate 34 is also eliminated in the structure shown in FIG. 2(a).Thus, the number of components is reduced in the structure shown in FIG.2(a), and the man-hour for assembling is also reduced. In addition, adimension in the radial direction of the long thin centrifugal fan canbe reduced compared with the structure shown in FIG. 1.

FIG. 2(b) is an enlarged cross section of the sleeve 31 and therotational shaft 33 in the structure shown in FIG. 2(a). As understoodfrom this diagram, the rotational shaft 33 has a reduced diameterportion 33 a at a middle portion in the axial direction of the partsliding with the sleeve 31, and the reduced diameter portion 33 a hasits diameter that is slightly smaller than other portion. Thus, an areaof the sliding surface is reduced. Namely, there is no sliding action atthe reduced diameter portion 33 a between the rotational shaft 33 andthe sleeve 31, and the sliding action occurs only at the parts at bothsides of the reduced diameter portion 33 a (i.e., a bottom side part 33b and a top side part 33 c). As a result, a bearing loss due to thesliding friction is reduced.

Note that although it is possible to cut a part of the inner surface ofthe sleeve 31 instead of cutting the rotational shaft 33 (forming thereduced diameter portion 33 a) for reducing an area of the slidingsurface, cutting of the rotational shaft 33 is easier than cutting ofthe inner surface of the sleeve 31. Particularly, if the rotationalshaft 33 is made of a metal, cutting of the rotational shaft 33 is notdifficult. After forming the reduced diameter portion 33 a by cuttingthe rotational shaft 33, the above-mentioned surface hardening processor coating of the sliding surface may be performed so as to increase itshardness.

In this way, the method of forming the reduced diameter portion 33 a onthe rotational shaft 33 at the part sliding with the sleeve 31 so as toreduce an area of the sliding surface for reducing a bearing loss can beapplied to the structure of the first embodiment shown in FIG. 1. Inaddition, the method can also be applied to a structure with rotationalsleeve (fixed shaft) that will be described later, namely a structure inwhich the shaft member is not the rotational shaft but a fixed shaft.

In the structure shown in FIG. 3(a), the sleeve 31 is divided into twomembers in the axial direction, and between them there is a felt tube 36that is means for keeping lubricating oil and is attached to therotational shaft 33. The felt tube 36 is impregnated with a lubricatingoil such as an ester lubricating oil having a coefficient of viscosityless than 0.02 Pa-s. Thus, the lubricating oil is supplied to and kepton the sliding surfaces of the rotational shaft 33 and the sleeve 31, sothat a bearing loss due to friction between the sliding surfaces can bereduced.

In the structure shown in FIG. 3(b), the step level (cut portion) of thereduced diameter portion 33 a that is formed on the rotational shaft 33in the structure shown in FIG. 2(b) is a little larger, and the felttube 36 that is means for keeping lubricating oil is attached to thatportion. Also in this case, the lubricating oil that is impregnated inthe felt tube 36 is supplied to and kept on the sliding surfaces of therotational shaft 33 and the sleeve 31 so that a bearing loss due tofriction between the sliding surfaces can be reduced similarly to thestructure shown in FIG. 3(a). Note that the means for keepinglubricating oil is not limited to the felt tube 36 as shown in FIGS.3(a) and 3(b). In addition, even if means for keeping lubricating oil isnot provided separately, it is possible to keep lubricating oil betweenthe sliding surfaces of the rotational shaft 33 and the sleeve 31 sothat the above-mentioned lubricating effect can be obtained during acertain period.

Embodiment 2

FIG. 4 is a cross sectional view showing a structure of a long thincentrifugal fan according to a second embodiment of the presentinvention. This long thin centrifugal fan includes a housing 11 having asubstantially cylindrical shape elongated in the axial direction, whichhouses an impeller 12 and a motor 13 for rotating the impeller 12. Theimpeller 12 is disposed mainly at the top side (the right side in FIG.4) in the axial direction within the housing 11, while the motor 13 isdisposed mainly at the bottom side (the left side in FIG. 4) in theaxial direction within the housing 11.

The impeller 12 includes a blade portion 14 (at the top side) havingplural blades of a shape elongated in the axial direction that arearranged at a predetermined pitch in the circumferential direction, anda bottom end portion 15 having a substantially cylindrical shape forsupporting the blade portion 14. The tip of the blade portion isprovided with a ring-like linking portion 16 for linking and supportingtip portions of the plural blades. When the impeller 12 rotates,external air is taken in through a suction opening 17 provided to thetip portion in the axial direction of the housing 111 as illustrated byan arrow IN. Then, the air is blown out through an outlet opening thatis provided to a part in the circumferential direction of the housing11.

The motor 13 has a structure in which a torque generating portion 13 aincluding a stator armature 21 and a rotor magnet 22, and a bearingportion 13 b that retains a rotational member in a rotatable manner arearranged tandem in the axial direction. In addition, the bearing portion13 b is disposed between the impeller 12 and the torque generatingportion 13 a. According to this structure, it is possible to reduce adimension in the radial direction can be reduced compared with thestructure of the first embodiment shown in FIG. 1. Namely, although anouter diameter of the motor 13 depends on a total sum of the outerdiameter of the bearing portion 13 b and a size (thickness) of thetorque generating portion 13 a in the radial direction in the structureshown in FIG. 1, the outer diameter of the motor 13 depends on eitherthe outer diameter of the bearing portion 13 b or the size (outerdiameter) of the torque generating portion 13 a that is larger one thanthe other (the outer diameter of the torque generating portion 13 a inthe embodiment shown in FIG. 4) in the structure shown in FIG. 4. As aresult, a dimension in the radial direction of the long thin centrifugalfan can be reduced.

More specifically, when plural members rotating as one unit bodyincluding the impeller 12 and the rotational member (the rotor yoke 25)are regarded as one element, the bearing portion 13 b (i.e., theintermediate point thereof in the axial direction) is disposed at abarycenter of the element or at vicinity of the barycenter. According tothis structure, rotation of the impeller 12 can be stabilized easily.Namely, vibration of the impeller 12 accompanying its rotation can bereduced, and a load to the bearing portion can be suppressed so that along life can be realized. This structure is useful particularly in thecase where the impeller 12 is rotated at a high rotation speed.

In the case of this embodiment too, the bearing portion 13 b includesthe rotational shaft 33 and the sleeve 31 having a cylindrical shapethat engages the outer surface of the rotational shaft 33 with aclearance, and both the rotational shaft 33 and the sleeve 31 are madeof a ceramic. However, it is possible that one of the rotational shaft33 and the sleeve 31 is made of a metal, on which a surface hardeningprocess or coating of the sliding surface is performed so as to increaseits hardness as mentioned in the first embodiment. The rotational shaft33 is fixed to the rotor yoke 25 that is the rotational member, and thesleeve 31 is engaged with and fixed to the inner surface of the sleeveholder 32.

The rotor yoke 25 having a substantially cylindrical shape is elongatedin the axial direction and extends from the torque generating portion 13a to the bearing portion 13 b. As understood from the cross sectionshown in FIG. 4, the rotor yoke 25 is provided with step portions sothat its diameter decreases from the bottom side to the top side bythree steps. In other words, the rotor yoke 25 has a large diameterportion 25 a, a medium diameter portion 25 b and a small diameterportion 25 c. For example, a plurality of the rotor magnets 22 isarranged on the inner surface of the large diameter portion 25 a of therotor yoke 25 at a predetermined pitch in the circumferential direction.Each of the rotor magnets 22 is opposed to the stator armature 21 with aconstant gap.

The bottom end portion 15 of the impeller 12 is engaged with and fixedto the outer surface of the medium diameter portion 25 b and the smalldiameter portion 25 c of the rotor yoke 25. The rotational shaft 33 isfixed to the small diameter portion 25 c of the rotor yoke 25 by pressfitting. Therefore, the rotational shaft 33, the rotor yoke 25 and theimpeller 12 rotate as one unit body. In addition, as understood fromFIG. 4, the top end portion 33 d of the rotational shaft 33 is disposedwithin the inner space of the blade portion 14 of the impeller 12.Namely, the rotational shaft 33 extends from the inside of the bottomend portion 15 of the impeller 12 to the inside of the blade portion 14in the axial direction. Accompanying with this, the small diameterportion 25 c of the rotor yoke 25 also extends from the inside of thebottom end portion 15 of the impeller 12 to the inside of the bladeportion 14. Thus, sufficient length in the axial direction for securefixing and aligning between the rotational shaft 33 and the rotor yoke25 is ensured, while a dimension in the axial direction of the long thincentrifugal fan can be reduced.

In addition, the sleeve holder 32 that is made of a metal includes acylindrical portion 32 a for engaging with the outer surface of thesleeve 31 so as to retain the same and a shaft portion 32 b that extendsfrom the center of the bottom end surface of the cylindrical portion 32a toward the bottom side. The axis of the cylindrical portion 32 aagrees the axis of the shaft portion 32 b. The shaft portion 32 b fitsin the stator armature 21, and further the bottom end portion thereoffits in the center through-hole of the base member 24 made of a resin(or a metal) and fixed to the same. The base member 24 is fixed to theinner wall of the bottom side of the housing 11 made of a resin (or ametal). Thus, the sleeve holder 32 is fixed so that the axis of thesleeve holder 32 agrees the center axis of the substantially cylindricalhousing 11.

In the cylindrical portion 32 a of the sleeve holder 32, the innersurface at the bottom side is provided with a thrust plate 34 made of ametal to which the bottom end portion of the rotational shaft 33 canabut. Thus, a thrust bearing is constituted. If the rotational shaft 33is made of a ceramic, it is preferable that the thrust plate 34 is madeof a ceramic too or a metal having enhanced surface hardness by thesurface hardening process or the like as described above. Alternatively,it is possible to keep the bottom end portion of the rotational shaft 33as a free end so that the thrust plate 34 can be eliminated. In thiscase, air dumping or oil dumping may be used for retaining the bottomend portion of the rotational shaft 33. In addition, a seal member isprovided for sealing a ring-like gap between the opening of the sleeveholder 32 at the top side and the rotational shaft 33, so as to preventdust from entering the inside of the cylindrical portion 32 a of thesleeve holder 32.

Note that the structure of the bearing portion that is described as avariation of the first embodiment can be applied to this secondembodiment, too. Namely, as shown in FIGS. 2 and 3, the sleeve 31 andthe sleeve holder 32 can be made as one unit member, and/or the reduceddiameter portion 33 a can be formed on a part of the rotational shaft 33so that an area of the sliding surface (i.e., the bearing loss) can bereduced, and/or means for keeping lubricating oil that is supplied tothe sliding surface can be provided.

Embodiment 3

FIG. 5 is a cross sectional view showing a structure of a long thincentrifugal fan according to a third embodiment of the presentinvention. This long thin centrifugal fan includes a housing 11 having asubstantially cylindrical shape elongated in the axial direction, whichhouses an impeller 12 and a motor 13 for rotating the impeller 12. Astructure of the impeller 12 is the same as the embodiments describedabove, so overlapping description is omitted here.

A motor 13 of the long thin centrifugal fan in this embodiment issimilar to that of the second embodiment. Namely, the motor 13 has astructure in which a torque generating portion 13 a including a statorarmature 21 and a rotor magnet 22, and a bearing portion 13 b thatretains a rotational member in a rotatable manner are arranged tandem inthe axial direction, and a bearing portion 13 b is disposed between theimpeller 12 and the torque generating portion 13 a. An effect of thisstructure is also the same as described in the second embodiment. Inaddition, when plural members rotating as one unit body including theimpeller 12 and the rotational member (the rotor yoke 25) are regardedas one element, the bearing portion 13 b (i.e., the intermediate pointthereof) is disposed at a barycenter of the element or at vicinity ofthe barycenter. This structure and its effect are also the same asdescribed in the second embodiment.

The long thin centrifugal fan of this embodiment has a bearing portion13 b of the motor 13 whose structure is substantially different from theembodiments described above. Namely, unlike the embodiments describedabove in which the bearing portion 13 b has a rotational shaft (fixedsleeve) structure, the bearing portion 13 b in the third embodiment hasa rotating sleeve (fixed shaft) structure. In other words, although thebearing portion 13 b in this embodiment is also a sleeve bearingincluding a shaft member and a sleeve having a cylindrical shape thatengages the outer surface of the shaft with a clearance, the shaftmember is a fixed shaft 33 that is fixed to a base 24, and a sleeve 31is fixed to a rotor yoke 25 that is a rotational member via a sleeveholder 32. Note that although there is a difference between the fixedside and the rotating side, the fixed shaft that is the shaft member isdenoted by the reference numeral 33 that is the same as the rotationalshaft in the embodiments described above. Other members including thesleeve 31 and the sleeve holder 32 are also denoted by the samereference numerals as the embodiments described above.

In FIG. 5, the rotor yoke 25 having a substantially cylindrical shapeextends from the torque generating portion 13 a to the bearing portion13 b in the axial direction. The rotor yoke 25 has step portions so thatits diameter decreases from the bottom side to the top side by threesteps. Namely, the rotor yoke 25 has a large diameter portion 25 a, amedium diameter portion 25 b and a small diameter portion 25 c. Aplurality of the rotor magnets 22 is arranged on the inner surface ofthe large diameter portion 25 a of the rotor yoke 25 at a predeterminedpitch in the circumferential direction. Each of the rotor magnets 22 isopposed to the stator armature 21 with a constant gap.

The bottom end portion 15 of the impeller 12 is engaged with and fixedto the outer surface of the medium diameter portion 25 b and the smalldiameter portion 25 c of the rotor yoke 25. The small diameter portion25 c of the rotor yoke 25 has a closed end surface at the top side,which seals the opening of the impeller 12 at the boundary between theblade portion 14 and the bottom end portion 15. A cylindrical sleeveholder 32 having a closed end surface at the top side is engaged withand fixed to the inner surface of the medium diameter portion 25 b andthe inner end surface of the small diameter portion 25 c of the rotoryoke 25, while a cylindrical sleeve 31 is fixed to the inner surface ofthe sleeve holder 32.

The top side of the fixed shaft 33 that is the shaft member is engagedwith the inner surface of the sleeve 31 with a clearance, and the bottomside of the fixed shaft 33 fits in the stator armature 21 and furtherfits in and fixed to the center through-hole of the base member 24 madeof a resin (or a metal). The base member 24 is fixed to the inner wallof at the bottom side of the housing 11 made of a resin (or a metal).Thus, the fixed shaft 33 is fixed so that the axis of the fixed shaft 33agrees the center axis of the substantially cylindrical housing 11, andthe sleeve 31, the sleeve holder 32, the rotor yoke 25 and the impeller12 rotates as one unit body around the top side of the fixed shaft.

The inner surface of the sleeve holder 32 at the top side is providedwith a thrust plate 34 made of a metal to which the top end portion ofthe fixed shaft 33 can abut. Thus, a thrust bearing is constituted. Ifthe fixed shaft 33 is made of a ceramic, it is preferable that thethrust plate 34 is made of a ceramic too or a metal having enhancedsurface hardness by the surface hardening process or the like asdescribed above. Alternatively, it is possible to keep the bottom endportion of the rotational shaft 33 as a free end so that the thrustplate 34 can be eliminated. In this case, air dumping or oil dumping maybe used for retaining the bottom end portion of the rotational shaft 33.In addition, a seal member is provided for sealing a ring-like gapbetween the opening of the sleeve holder 32 at the bottom side and thefixed shaft 33, so as to prevent dust from entering the inside of thesleeve holder 32.

The rotating sleeve (fixed shaft) bearing structure of this embodimentcan be realized easily by structure in which the torque generatingportion 13 a and the bearing portion 13 b for retaining the rotationalmember in a rotatable manner are arranged tandem in the axial direction,and the bearing portion 13 b is disposed between the impeller 12 and thetorque generating portion 13 a. In addition, use of the sleeve made of aceramic also contributes largely to realizing the structure. Although asleeve bearing using a sleeve made of a sintered metal impregnated withlubricating oil has been used conventionally, it is difficult in thiscase to realize the rotating sleeve structure because lubricating oilmoves to the outer side by the centrifugal force when the sleeve rotatesand lack of lubricating oil occurs on the inner sliding surface. Incontrast, if a sleeve made of a ceramic is used as a ceramic bearing,such a problem does not happen so that the rotating sleeve structure canbe realized easily.

Note that variations of the bearing portion described in the firstembodiment can be applied to this third embodiment, too. Namely, asshown in FIGS. 2 and 3, the sleeve 31 and the sleeve holder 32 can bemade as one unit member, and/or the reduced diameter portion 33 a can beformed on a part of the rotational shaft 33 so that an area of thesliding surface (i.e., the bearing loss) can be reduced, and/or meansfor keeping lubricating oil that is supplied to the sliding surface canbe provided.

Note that if both the sleeve and the shaft are made of a ceramic, it ispreferable to use the same ceramic material for making them. Although aceramic is an insulator, generation of static electricity due toelectrification of the surface can be prevented by using the same typeof ceramic for the sliding surfaces. If static electricity is generated,the bearing may adsorb dust or micro particles, which may deteriorateperformance of the bearing. According to the present invention, the illeffect of dust or micro particles to the bearing can be prevented, sothat a long life of the centrifugal fan can be secured. The presentinvention is useful especially in an environment with much dust.

In addition, a plurality of shallow grooves may be formed on at leastone of the inner surface of the sleeve and the outer surface of theshaft member that are sliding surfaces of the bearing. The shallowgroove is preferably a linear groove extending in the vertical direction(axial direction) or a groove having a herringbone shape, a screw-likeshape (a spiral shape) or a curved shape. As a plurality of grooveshaving these shapes is arranged, pressure varies along the axialdirection so that stiffness of the bearing is enhanced at a part of highpressure when relative rotation between the sleeve and the shaft memberis generated. Therefore, contact between the sleeve and the shaft membercan be prevented, and stability when an external force is applied can beincreased. Note that a set of grooves may be provided, or plural sets ofgrooves may be provided separately in the axial direction. The groovescan be formed by coining, pressing, electrochemical machining, cuttingor other processes.

When the grooves are formed, bearing stiffness is enhanced, but a shaftloss increases. If the centrifugal fan is very small and if the fluidthat generates dynamic pressure is a gas, increase of the shaft loss dueto forming of the grooves is not so large. If the shaft loss isoutstandingly large, it is preferable to decrease an outer diameter ofthe shaft in parts where the dynamic pressure is not increased (as thestructure described in claim 2) so that the shaft loss can be reducedwithout lowering the stiffness.

Although embodiments and variations of the present invention aredescribed above, the present invention is not limited to theseembodiments and variations but can be embodied variously. In addition,materials and shapes of the members shown in the above description aremerely embodiments, and it should not be interpreted that the structureof the present invention is limited to the materials and the shapes.

While example embodiments of the present invention have been shown anddescribed, it will be understood that the present invention is notlimited thereto, and that various changes and modifications may be madeby those skilled in the art without departing from the scope of theinvention as set forth in the appended claims and their equivalents.

1. A centrifugal fan comprising: an impeller including a blade portionhaving plural elongated blades arranged at a predetermined pitch in thecircumferential direction; and a motor for rotating the impeller,wherein the impeller and the motor are arranged tandem in the axialdirection, a shaft having a cylindrical outer peripheral surface, asleeve having a cylindrical inner peripheral surface opposed to theouter peripheral surface of the shaft in the radial direction with aminute gap, a bearing portion that constitutes the motor including saidshaft and said sleeve, wherein one of the shaft and the sleeve is madeof a ceramic, and the other is made of a ceramic or a metal.
 2. Thecentrifugal fan according to claim 1, wherein the impeller is rotated ata rotation speed more than or equal to 15,000 revolutions per minute. 3.The centrifugal fan according to claim 1, wherein the sleeve and theshaft are both made of the same type of ceramic.
 4. The centrifugal fanaccording to claim 2, wherein the sleeve and the shaft are both made ofthe same type of ceramic.
 5. The centrifugal fan according to claim 1,wherein a reduced diameter portion is formed on the shaft member at amiddle portion in the axial direction of the sliding portion with thesleeve, the reduced diameter portion having a diameter a little smallerthan other portions.
 6. The centrifugal fan according to claim 2,wherein a plurality of shallow grooves is formed on at least one of theinner surface of the sleeve and the outer surface of the shaft member.7. The centrifugal fan according to claim 3, wherein a plurality ofshallow grooves is formed on at least one of the inner surface of thesleeve and the outer surface of the shaft member.
 8. The centrifugal fanaccording to claim 4, wherein a plurality of shallow grooves is formedon at least one of the inner surface of the sleeve and the outer surfaceof the shaft member.
 9. The centrifugal fan according to claim 1,wherein among elements that constitute the motor, a torque generatingportion including an armature and a field magnet, and a bearing portionare arranged tandem in the axial direction.
 10. The centrifugal fanaccording to claim 1, wherein the shaft member is fixed to a rotationalmember, and the sleeve that retains the shaft member in a rotatablemanner is fixed to the housing directly or via a base member.
 11. Thecentrifugal fan according to claim 9, wherein the shaft member is fixedto a rotational member, and the sleeve that retains the shaft member ina rotatable manner is fixed to the housing directly or via a basemember.
 12. The centrifugal fan according to claim 1, wherein a sleevemade of a ceramic is fixed to a rotational member, and a shaft member isfixed to the housing directly or via a base member.
 13. The centrifugalfan according to claim 9, wherein a sleeve made of a ceramic is fixed toa rotational member, and a shaft member is fixed to the housing directlyor via a base member.
 14. The centrifugal fan according to claim 13,wherein the top side of the shaft member is disposed within the innerspace of the blade portion of the impeller.
 15. The centrifugal fanaccording claim 10, wherein the bottom side of the shaft member is afree end without a thrust bearing.
 16. The centrifugal fan according toclaim 1, wherein when plural members rotating as one unit body includingthe impeller and the rotational member are regarded as one element, thebearing portion is disposed at a barycenter of the element or atvicinity of the barycenter.
 17. The centrifugal fan according to claim2, wherein when plural members rotating as one unit body including theimpeller and the rotational member are regarded as one element, thebearing portion is disposed at a barycenter of the element or atvicinity of the barycenter.
 18. The centrifugal fan according to claim9, wherein when plural members rotating as one unit body including theimpeller and the rotational member are regarded as one element, thebearing portion is disposed at a barycenter of the element or atvicinity of the barycenter.
 19. The centrifugal fan according to claim1, wherein means for keeping lubricating oil that is supplied to thesliding surfaces of the shaft member and the sleeve are attached to theshaft member or the sleeve.